Wireless mobile Internet is a next-generation communication method that adds motility to a short-distance data communication method using fixed access points, such as wireless LAN. Various standards for the wireless mobile Internet have been proposed, and international standardization of mobile Internet such as IEEE 802.16e has been carried out.
FIG. 1 illustrates the concept of wireless mobile Internet. A wireless portable Internet system includes a subscriber station 10, base stations 20 and 21 carrying out wireless communication with the subscriber station 10, routers 30 and 31 respectively connected to the base stations 20 and 21 through gateways, and the Internet.
A conventional wireless LAN method such as IEEE 802.11 provides a data communication method enabling wireless data communication in a LAN based on fixed access points. However, the convention wireless LAN method does not provide mobility of a mobile subscriber station and supports only wireless LAN data communication.
The wireless portable Internet system, which is being developed by the IEEE 802.16 group, secures mobility of the subscriber station 10 shown in FIG. 1 even when the subscriber station 10 moves from a cell managed by the base station 20 to a cell managed by the base station 21, to provide data communication service that does not interrupt a service session.
IEEE 802.16e is a standard supporting a metropolitan area network (MAN), which means an information communication network covering a middle-size area between an area managed by a LAN and an area covered by a WAN. Accordingly, the wireless portable Internet system supports handover of the subscriber station 10 as a mobile communication service does, and executes dynamic IP address allocation when the subscriber station is moved.
Here, the subscriber station 10 communicates with the base stations 20 and 21 using an orthogonal frequency division multiple access (OFDMA) method. The OFDMA method is a combination of a frequency multiplexing method using a plurality of orthogonal frequency subcarriers as a plurality of subchannels, and a time division multiplex (TDM) method. The OFDMA method is robust against fading generated in a multi-path, and has a high data transfer rate.
Furthermore, IEEE 802.16e employs an adaptive modulation and coding (AMC) method such that modulation and coding are adaptively selected between the subscriber station 10 and the base stations 20 and 21 according to request/acceptance.
FIG. 2 illustrates a hierarchical structure of the wireless portable Internet system. The hierarchical structure of the IEEE 802.16e wireless portable Internet system includes a physical layer L10 and a media access control (MAC) layer. The physical layer L10 is in charge of a wireless communication function, such as modulation, demodulation, and coding, carried out by a conventional physical layer.
In the wireless portable Internet system, a single MAC layer manages various functions, distinguished from a wired Internet system having layers subdivided by functions. The MAC layer includes a privacy sublayer L21, a MAC common part sublayer L22, and a service specific convergence sublayer L23. The service specific convergence sublayer L23 is in charge of payload header suppression and QoS mapping in continuous data communication. The MAC common part sublayer L22, which is an essential part of the MAC layer, processes system access, bandwidth allocation, connection setup, and maintenance, as well as QoS management. The privacy sublayer L21 carries out device authentication, security key exchange, and coding. The privacy sublayer L21 performs only the device authentication, and an upper layer (not shown) of the MAC layer carries out user authentication.
FIG. 3 illustrates a connection structure of a base station and a mobile subscriber station in the wireless portable Internet system. A connection C1 exists between the MAC layer of the subscriber station and the MAC layer of the base station. The term connection means a logical connection relationship, not a physical connection relationship, and it is defined as a mapping relationship between the MAC peers of the subscriber station and the base station for transmitting single service flow traffic. Accordingly, a parameter or a message defined on the connection C1 defines functions of the MAC peers. Actually, the parameter or message is processed into a frame and transmitted through a physical layer. Then, the frame is analyzed and a MAC layer carries out a function corresponding to the parameter or message.
A MAC message transmitted through the connection C1 includes a connection identifier (CID) that is a MAC layer address for identifying the connection; a MAP that defines symbol offsets and subchannel offsets of bursts, time-divided by the subscriber station on a downlink/uplink, and the number of symbols and subchannels of an allocated resource; and a downlink/uplink channel descriptor (DCD/UCD) that describes characteristics of a physical layer based on downlink/uplink characteristics. In addition, the MAC message includes various messages executing request (REQ), response (RSP), and acknowledgement (ACK) functions with respect to various operations.
FIG. 4 illustrates a frame structure of the wireless portable Internet system. Referring to FIG. 4, a frame is divided into a downlink frame F1 and an uplink frame F2 based on a transmission direction. The vertical axis of the frame means a subchannel, and its horizontal axis is a time axis.
The downlink frame F1 includes a preamble, a downlink MAP, an uplink MAP, and a plurality of downlink bursts. The downlink bursts are obtained by classifying a channel or a resource by transmission levels having the same modulation method or channel coding, not by users. Accordingly, the downlink MAP includes offset information, modulation method information, and coding information with respect to multiple users using the same modulation method and channel coding to perform resource application to the users. The MAP has a broadcast channel character and requires high robustness.
In the case of the uplink frame F2, data transmission is carried out for each user, and uplink bursts include user information.
FIG. 5 is a flow chart showing a connection setting process in the wireless portable Internet system.
When the subscriber station enters the area of the base station in the step S1, the base station sets downlink synchronization with the subscriber station in the step S2. When the downlink synchronization is set, the subscriber station acquires an uplink parameter in the step S3. For example, the parameter can include a channel descriptor message based on a characteristic of a physical layer (signal-to-noise ratio, for instance).
In the step S5, ranging between the subscriber station and the base station is carried out. Ranging is a process of matching timing, power, and frequency information of the subscriber station with those of the base station. Initial ranging is carried out first, and then periodic ranging is performed. When the ranging process is finished, negotiation about basic service providing capability for setting up the connection between the subscriber station and the base station is carried out in the step S5. When the negotiation is completed, the subscriber station is authenticated using a device identifier such as a MAC address of the subscriber station and a note of authentication in the step S6.
When the authentication of the subscriber station is accomplished so that the subscriber station is confirmed to be authorized to use the wireless portable Internet system, a device address of the subscriber station is registered in the step S7, and an IP address management system such as a DHCP server provides an IP address to the subscriber server to set up an IP connection in the step S8. In the step S9, the subscriber station provided with the IP address carries out connection setup for transmitting data.
In the meantime, the above-described wireless portable Internet system does not execute communication only at a fixed place, but it has mobility as high as a MAN level. Thus, the subscriber station uses a battery as a power supply means. Accordingly, battery use time becomes a large factor restricting utilization time in the wireless portable Internet system.
Therefore, the wireless portable Internet system such as IEEE 802.16e proposes a slip mode in order to save battery power. The slip mode allows the subscriber station to be in a sleep state for a sleep interval when there is no data transmitted to the subscriber station to save power of the subscriber station. When the subscriber station enters the sleep state, the subscriber station does not perform any operation for receiving data for the sleep interval.
When the sleep interval is finished, the subscriber station is converted into a listening mode to confirm whether there is data standing by ready to be transmitted to the subscriber station.
FIG. 6 is a flow chart showing a sleep mode operation in the wireless portable Internet system.
When the subscriber station wants to enter the sleep mode, the subscriber station should be approved by the base station. Accordingly, the subscriber station 10 that wants to enter the sleep mode sets a sleep interval and requests the base station 20 to approve the sleep mode (S10). When there is a sleep mode request, the base station designates a sleep interval and approves the sleep mode (S11). Then, the subscriber station enters the sleep mode state where the subscriber station does not receive any data from the moment M of time of entering the sleep mode (S12). When the initial sleep interval is finished, the subscriber station is converted into the listening mode to confirm whether there is data standing by ready to be transmitted from the base station to the subscriber station (S13). When there is no data standing by during the initial sleep interval, the base station sets a message representing presence of data traffic to 0 and transmits the message to the subscriber station (S14).
When the subscriber station confirms that there is no data transmitted thereto during the listening mode, the subscriber station enters the sleep mode again (S15). Here, a sleep interval can be identical to or longer than the initial sleep interval.
When there is data standing by ready to be transmitted to the subscriber station during the second sleep interval, the base station can buffer the data traffic (S17). The subscriber station is informed of the presence of the buffered data traffic when the subscriber station is in the listening mode (S18). When the subscriber station confirms that there is data traffic to be transmitted thereto in the listening mode (S16), the subscriber station finishes the sleep mode and enters an awake mode to receive the buffered data traffic and carry out data communication with the base station.
According to the aforementioned sleep mode operation, the subscriber station is in the sleep state continuously as long as there is no data to be transmitted to the subscriber station. Thus, unnecessary power consumption is prevented. In the wireless portable Internet system, however, data is lost in case of handover due to problems generated when the sleep mode or handover is executed.
FIG. 7 illustrates a handover process in the wireless portable Internet system. The wireless portable Internet system performs hard handover and backward handover because it carries out data transmission and reception. The backward handover means that a serving base station receives a handover request from a subscriber station to process handover. In this case, stable handover can be executed because the serving base station previously has information about the subscriber station.
A detailed handover method will now be explained. The subscriber station 10 transmits a handover request message HO/REQ that is a MAC message to the serving base station BS1 in order to carry out handover. The serving base station BS, which has received the handover request message HO/REQ, checks whether an adjacent base station can accept handover of the subscriber station 10, and then transmits a list of at least one target base station enabling handover to the subscriber station through a handover response message HO/RSP.
The subscriber station 10 receives the handover response message HO/RSP, selects a target base station BS2 from the destination base station list, informs the serving base station of the selected target base station BS2, and then attempt re-entry into a network via the target base station BS2.
FIG. 8 illustrates a drop state generated in a conventional wireless portable Internet system.
As described above, the subscriber station uses the sleep mode to reduce power consumption in the wireless portable Internet system. When handover is required while the subscriber station is in the sleep state, communication between the subscriber station and the serving base station is interrupted. Furthermore, communication between the subscriber station and the serving base station may be interrupted before handover of the subscriber station with the serving base station is finished.
After this drop phenomenon, when the subscriber station attempts re-entry into a network via the target base station BS2, the target base station BS2 cannot perform an initial networking process and must set up a required wireless channel because it does not have information about the subscriber station. Accordingly, a service session is not maintained. Moreover, the data buffered by the serving base station BS for the subscriber station is lost.